The present invention relates in general to internally doping a tube of glass, i.e. incorporating one or more dopants in an inside layer of the tube, with the term "dopant" being used herein to designate any material capable of locally modifying the characteristics of the glass. The invention is particularly applicable to cases where it is not desired or not possible to include a volatile precursor of such an element in a mixture of reaction gases. The invention then meets a need which is currently felt in the manufacture of optical fibers. Various dopants may be used in such manufacture for the purpose of locally modifying the optical characteristics of a tube of glass which is subsequently hot-collapsed in order to transform it into a preform which can be hot-drawn in order to provide a fiber. When the dopant used has a volatile precursor, it is generally easy to include the precursor into the mixture of reaction gases from which the dopant can subsequently be incorporated in the glass by conventional methods. However, these methods are no longer applicable when the dopant to be incorporated does not possess a volatile precursor. This applies to rare earths such as neodymium or erbium. When the invention is applied to the manufacture of optical fibers, the glass in which the dopant is to be incorporated is silica glass.
In optical fiber manufacture, the conventional doping precursors used in the manufacture of preforms by the so-called prior art "MCVD" method have vapor pressures of a few thousand Pascals even at ambient temperature, thereby making it possible in a first prior art method to incorporate them by bubbling a carrier gas through a reactor in order to constitute said mixture of reaction gases. However, the compounds of rare earths which can be used for manufacturing optical fibers for constituting lasers or sensors have very low vapor pressures even at temperatures close to their melting points, for example:
NdCl.sub.3 3.52 Pascals at 820.degree. C.; and PA1 ErCl.sub.3 14.84 Pascals at 830.degree. C.
This first method including said bubbling is then no longer usable. The first preforms to be doped with rare earths were made using a second prior art method referred to as the "doping chamber" method (see document No. 1 listed in the bibliography at the end of the description). When this method is used, the incorporation of the dopant is not as uniform longitudinally as could be desired.
A third prior art method is described in European patent application No. 0 025 230, but for incorporating dopants other than rare earths. This document proposes incorporating the dopant in a precursor which is dissolved in a liquid carrier, mixing the precursor and the liquid carrier with a precursor for silica dissolved in a liquid carrier, conveying this liquid mixture under pressure via a delivery tube to a point of application which is displaceable inside a glass tube blank having an inside layer into which the dopant is to be incorporated, while also conveying a liquid reagent under pressure to the same point for the purpose of reacting chemically with said precursors in order to form the dopant and silica, spraying said precursors and their carrier liquids together with said reagant towards the inside surface of the glass tube blank, and displacing the point of application so as to sweep said inside surface, thereby forming a layer of doped glass at low temperature, with the liquid carriers subsequently being elminated by drying.
This third prior art method does not seem to be usable in practice since the layer of glass formed contains such a quantity of said liquid carriers at the moment of formation that when the layer is dried and subsequently consolidated by heating, it does not achieve acceptable optical qualities, in particular concerning transparency.
A fourth prior art method in the same technical field includes various operations which are common, at least in some respects, both to said prior art method and to the method of the present invention, at least in a typical implementation of the invention.
These two methods begin with a tube blank which is not doped or which is at least not completely doped with the dopant to be incorporated.
Said common operations are the following:
making a tube blank constituted by glass and having a length between a first end and a second end, and having an inside surface about an inside space;
making a liquid doping liquor constituted by a volatile carrier liquid and containing a dopant suitable for performing said doping;
depositing said dopant in liquid form by depositing a quantity of said doping liquor on the inside surface of said tube blank, with the quantity deposited being small enough to prevent said liquor from running after being deposited;
drying said inside surface after depositing said dopant by evaporating said liquid carrier;
depositing a covering layer of glass on said inside surface of said tube blank after said operation of drying said surface; and
heating-diffusion, said operation comprising heating said tube blank and said covering layer after said layer has been deposited at least partially in order to raise said tube and said layer to a diffusion temperature for diffusing said dopant into the glass.
In this fourth prior art method, said operation of depositing a covering layer takes place at a temperature such as to simultaneously constitute said operation of heating-diffusion. Said operation of depositing said dopant takes place in two stages. In a first stage a uniform porous inside layer (a "soot") is formed on the inside surface of the tube blank. In the second stage, said porous layer is impregnated with said doping liquor which is thus held uniformly over all of said surface. After drying, heating transforms said porous layer into a layer of glass containing the dopant. This fourth prior art method is described in document No. 2.
It suffers from the following drawbacks:
Impregnating the porous layer requires the tube blank to be removed from the glass-makers' lathe onto which the tube was previously mounted for prior deposition operations, in particular for forming said porous layer. The tube then needs to be remounted on the lathe for subsequent deposition operations or at least for the collapsing operation which transforms the tube into a preform from which an optical fiber can be drawn.
This gives rise to the difficult and expensive operation of dismounting and remounting the tube. As a result there is above all a danger of polluting the inside of the tube, in particular by soots which are deposited at the downstream end of the tube during earlier depositing operations. These soots are formed from a mixture of reaction gases running along the tube for depositing a layer of glass therein. They are constituted by particles which are not deposited at appropriate locations for forming said layer of glass and which have been entrained downstream by the remainder of the mixture of reaction gases. Reinserting them into the tube, even in very small quantities, while it is being dismounted or remounted, seriously degrades the optical qualities of the optical fiber which will eventually be made.
The objects of the present invention include, in particular, making it possible to introduce a dopant into a glass tube from one end only thereof, to avoid dismounting the tube once it has been mounted on a glass-makers' lathe, to avoid the danger of polluting said tube when performing said incorporation from the other end of the tube if said other end has already been polluted, to provide longitudinally uniform distribution of the dopant, and finally, when the tube is intended to be collapsed to constitute a preform from which an optical fiber is to be drawn, to make it possible to cheaply obtain an optical fiber having good optical qualities.